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Tin oxide is an n-type semiconductor material with a high covalent behavior. Mass transport in this oxide depends on the surface state promoted by atmosphere or by the solid solution of a non-isovalent oxide doping. The sintering and grain growth of this type of oxide powder is then controlled by atmosphere and by extrinsic oxygen vacancy formation. For pure SnOl powder the surface state depends only on the interaction of atmosphere molecules with the SnOz surface. Inert atmosphere like argon or helium promotes oxygen vacancy formation at the surface due to reduction of SnOz to SnO at the surface and liberation of oxygen molecules forming oxygen vacancies. As consequence surface diffusion is enhanced leading to grain coarsening but no densificdtion. Oxygen atmosphere inhibits the Sn02 reduction decreasing the surface oxygen vacancy concentration. Addition of dopants with lower valence at sintering temperature creates extrinsic charged oxygen vacancies that promote mass transport at grain boundary leading to densification and grain growth of this polycrystalline oxide.